This invention relates to radio frequency pulse generating devices and, in particular, to tunable microwave pulse generators.
It is known to product bursts of microwave energy by switching power across a gap or gaps in the central conductor of a coaxial line or wave guide as shown in U.S. Pat. No. 3,521,121 issued July 21, 1970 to J. M. Proud, Jr., U.S. Pat. No. 3,484,619 issued Dec. 16, 1969 to J. M. Proud, Jr., and U.S. Pat. No. 3,748,528 issued July 24, 1973 to H. Cronson. These devices are capable of providing high energy pulse bursts of microwave power up to approximately 10 GHz. In the waveguide device disclosed in the patent to H. Cronson, the center conductor of the coaxial line is provided with one or more switching gaps along its length and/or the end post spaced from the interior wall of the waveguide. A radio frequency (RF) block or impedance is disposed around the post adjacent the first switching gap with the gaps so dimensioned that energy switched by the first gap can pass the block or impedance, but oscillations caused by the discharge at the second gap cannot pass the block. The spark gap functions to steepen the leading edge of the travelling wave. The prior art structures possess several shortcomings which the instant invention overcomes. These shortcomings include, but are not limited to, poor efficiency, since the capacitance of the RF block, to be effective, must be relatively large compared to the distributed capacity of the end post. The capacitance of the RF block stores most of the initial energy and, upon discharge, tends to react with the lumped value of inductance in the post, resulting in low frequency oscillations which typically are less than one-half the desired operating frequency. In the Cronson waveguide device, mentioned hereinbefore, such oscillations lie below the waveguide cut-off frequency and would not be observed. However, the existence of such oscillations have been observed by the applicant using coaxial resonators operating in the lowest TEM mode with no cut-off frequency. This problem cannot be overcome by reducing the capacitance of the RF block since this would permit the microwave oscillations to leak out of the resonant structure.
Furthermore, the Cronson device requires a spark gap of relatively small dimensions to provide rapid charging of the end post. This causes the small electrode surface of the gap to wear rapidly under sparking causing rapid deterioration of the fast switching properties of the closely spaced spark gap. A sliding short is also used to adjust the coupling between the waveguide and the end post. However, it does not enable tuning of the generator since the microwave frequency generated is determined primarily by the cross-sectional dimensions of the waveguide.
The present invention is primarily concerned with a means for tuning or changing the resonant frequency of the waveguide, which heretofore was determined by the dimensions of the waveguide and the resonant element disposed therein.
Therefore, it is an object of the present invention to overcome the shortcomings of presently known radio frequency generators.
It is a further object of the present invention to provide a tunable highly efficient, high power radio frequency generator.
Another object of the present invention is to provide a simplified tunable radio frequency generator which converts DC energy into RF energy.
A still further object of the present invention is to provide a tunable microwave frequency generator which is small in size and relatively inexpensive to manufacture as compared to conventional RF microwave generators of equal output power.
The foregoing and other objects and advantages will appear from the description to follow. In the description, reference is made to the accompanying drawings which form a part thereof, and in which is shown by way of illustration a specific embodiment for practicing the invention. This embodiment will be described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense and the scope of the present invention is best defined by the appended claims.